Water base ink

ABSTRACT

[Problem] To provide an aqueous ink comprising a dispersant comprising a graft copolymer with excellent dispersibility of a dispersoid such as a pigment in an aqueous medium and is capable of maintaining a dispersion state stably for a prolonged period.  
     [Means for solution] Aqueous ink comprising, as a dispersant, a graft copolymer having a weight average molecular weight of from 1,000 to 100,000 and comprising: a polymer having a weight average molecular weight of from 300 to 10,000 represented by the following formula (1) or (2) as a side chain; and a polymer as a main chain comprising a hydrophobic monomer unit as an essential component, and, if necessary, other copolymerizable monomer unit as a constitutional component:  
                 
 
     wherein the sum of m and n is 1 or more, p is 0 or 1 or more, and R is a chemical structural unit having an ionic functional group:  
                 
 
     wherein q is 1 or more.

TECHNICAL FIELD

The present invention relates to aqueous ink containing a dispersantthat is low in dispersion viscosity and excellent in storage stability.

BACKGROUND ART

Dispersants comprising polymers are used for the purpose of well andstably dispersing each dispersoid for inorganic pigments such as calciumcarbonate, titanium oxide, etc., for drilling mud, for cement, or fordetergent builder. In aqueous inks for aqueous ballpoint pens and inkjetrecording, the following polymers are used as dispersants for dispersingpigments, which are dispersoids, in aqueous media.

According to the dispersants for organic pigments, it is important forthe polymers to have a function as a surfactant possessing both ahydrophobic region having affinity with dispersoids such as pigments anda hydrophilic region having affinity with aqueous media. As suchdispersants, graft copolymers comprising a hydrophobic macromonomercontaining an aromatic monomer such as styrene or alkyl(meth)acrylatesas a constituting unit and a hydrophilic monomer, and graft copolymerscomprising a hydrophilic macromonomer containing a monomer such as(meth)acrylic acid, or (meth)acrylic acid hydroxyalkyl ester and ahydrophobic monomer are known (refer to patent literature 1).

A pigment dispersant comprising a graft copolymer having a main chaincontaining a hydrophobic monomer and side chains derived frommacromonomers that are more hydrophilic than the main chain connectingto the main chain (refer to patent literature 2), and a carbon blackdispersant comprising a copolymer having a main chain containing afunctional monomer having reactivity with carbon black by chelatebonding and coordination bonding and an aromatic monomer and side chainscomprising polysiloxane, polyether, polyester and polyamide (refer topatent literatures 3 and 4), are also disclosed. In addition to theabove, there are also disclosed a pigment dispersant having lactone oroxyacid condensed product or polyalkylene glycol as side chains (referto patent literature 5), and an anionic polymer as a pigment dispersingassistant for offset printing ink, which is obtained by copolymerizing amonomer having an aromatic ring such as styrene and a monomer having ahydroxyl group such as hydroxyethyl methacrylate, and then adding acidanhydride to the hydroxyl group (refer to patent literature 6).

However, in the case of the aqueous inks prepared from the above graftcopolymers, dispersibility of a colorant is not sufficient and, inparticular, a dispersion state is liable to be broken by theenvironmental change such as temperatures. In addition, in the case ofthe pigment dispersants as described in patent literatures 1 and 2, inorder to dissolve the copolymers in an aqueous medium, it is necessaryto introduce a relatively large amount of ionic functional groups to thehydrophobic chains or lessen the hydrophobic components. Moreover, inthe case of the graft copolymers having nonionic hydrophilic side chainsas described in patent literatures 2 to 4, it is difficult to maintaindispersibility just after dispersion for a prolonged period, sinceelectric repulsion among dispersed particles cannot be obtained.

[Patent Literature 1]

JP-A-6-100810 (pp. 3-5) (The term “JP-A” as used herein refers to an“unexamined published Japanese patent application”.)

[Patent Literature 2]

JP-T-10-502097 (Claims) (The term “JP-T” as used herein refers to a“published Japanese translation of a PCT application”.)

[Patent Literature 3]

JP-A-9-59331 (Claims)

[Patent Literature 4]

JP-A-9-272706 (Claims)

[Patent Literature 5]

JP-A-60-161464 (Claims)

[Patent Literature 6]

JP-A-11-12528 (Claims)

DISCLOSURE OF THE INVENTION Problems that the Invention is to Solve

When a dispersoid such as a pigment is dispersed in an aqueous medium ofa dispersion medium, the structural design of a polymer having ahydrophobic group and a hydrophilic group is important for obtainingexcellent dispersibility. When the hydrophobic region of adsorbingmoiety onto a dispersoid and a hydrophilic moiety giving solubility intoa dispersion medium and electric repulsion of dispersoids are presenttoo close from the structural point of view of a polymer, it becomesdifficult to obtain good dispersibility in a dispersion medium, sincethey interfere with the other property or the hydrophilic region in apolymer is susceptible to the surface charge of a dispersoid. Therefore,it is necessary to make it difficult to be influenced from each other bythe intervention of a spacer such as long chain aliphatic hydrocarbonbetween the adsorbing region onto a dispersoid and a region havingaffinity with a dispersion medium in a polymer.

The present invention has been done for the purpose of the improvementof the drawbacks of the prior art. That is, the object of the inventionis to provide an aqueous ink containing a dispersant comprising a graftcopolymer with excellent dispersibility of a dispersoid such as apigment in an aqueous medium and capable of maintaining a dispersionstate stably for a long period of time.

MEANS FOR SOLVING THE PROBLEMS

As a result of earnest discussion to solve the above problems, thepresent inventors have found that a dispersoid such as a pigment isexcellent in dispersibility in an aqueous medium and a dispersion statecan be maintained stably for a prolonged period by the use of the graftcopolymer, as a dispersant, in which a side chain having ionic functiononly at a terminal in the side chain not bonding to the main chain isused as the side chain constituting the graft copolymer. Thus, theinvention has been achieved.

Incidentally, an acrylic acid or a methacrylic acid is described as a(meth)acrylic acid, and acrylate or methacrylate as (meth)acrylate.

ADVANTAGE OF THE INVENTION

A dispersant for use in the invention comprises a graft copolymer inwhich only the terminal in a side chain constituting the graft copolymernot bonding to a main chain has electric charge, so that the roles ofadsorbing performance onto a dispersoid such as a pigment and the likeand the affinity with a dispersion medium such as an aqueous medium canbe assigned to the side chain and the main chain, as a result aqueousink in which the functions of the main chain and the side chain do notinterfere with each other and a dispersoid such as a pigment is stablydispersed can be obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

The aqueous ink in the invention is composed of a dispersant comprisingthe graft copolymer, a colorant and an aqueous medium.

[1] Dispersant

A dispersant constituting aqueous ink in the invention comprises a graftcopolymer having a weight average molecular weight of from 1,000 to100,000 comprising, as a side chain, a polymer represented by thefollowing formula (1) or (2) having a weight average molecular weight offrom 300 to 10,000:

wherein the sum of m and n is 1 or more, p is 0 or 1 or more, and R is achemical structural unit having an ionic functional group;

wherein q is 1 or more;and as a main chain, a polymer comprising a hydrophobic monomer unit asthe essential constituent, and if desired, other copolymerizable monomerunit as the constituent.<1> Side Chain

The side chain polymer constituting the graft copolymer is representedby the following formula (1):

wherein the sum of m and n is 1 or more, p is 0 or 1 or more, and R is achemical structural unit having an ionic functional group.

In formula (1), as the ionic functional group bonded to R, anionicfunctional groups, e.g., a carboxyl group, a sulfonic acid group, and aphosphoric acid group, and cationic functional groups, e.g., primary,secondary, and tertiary amino groups, and a quaternary ammonium saltgroup can be exemplified.

R in formula (1) represents a chemical structural unit comprising achemical structure necessary to be bonded to a —(CO—C₅H₁₀O)— group inthe side chain polymer and an ionic functional group. As the specificexamples of the structures represented by R having a carboxyl group asthe anionic functional group, ring-opened structures of dibasic acidanhydrides, e.g., cyclic acid anhydrides, such as succinic acidanhydride, phthalic acid anhydride, 4-cyclohexene-1,2-dicarboxylic acidanhydride, cis-1,2-cyclohexanedicarboxylic acid anhydride, glutaric acidanhydride and trimellitic acid anhydride are exemplified.

As the cationic functional groups, amine structures and ammonium saltstructures via the above carboxyl groups and/or an epoxy group areexemplified, as described later.

The side chain polymer constituting the graft copolymer is furtherrepresented by the following formula (2):

wherein q is 1 or more.

The side chain may be comprised of one kind alone, or two or more kindsof side chains may be used.

The weight average molecular weight of the side chain constituting thegraft copolymer is from 300 to 1,000. When the weight average molecularweight is less than 300, the adsorption onto a dispersoid such as apigment and the dispersion in an aqueous medium are not well balanced,since the electric charges of the main chain and side chain terminalsare too close, as a result dispersion viscosity becomes too high anddispersibility with the lapse of time cannot be obtained. When itexceeds 10,000, electrostatic repulsion of side chain terminals weakensand dispersion stability with the passage of time worsens.

The chemical structure of the graft copolymer side chain represented byformula (1) or (2) are a polyalkylene oxide structure and/or a polyesterstructure. Specifically, polyethylene oxide, polypropylene oxide,poly(ethylene oxide/propylene oxide, a random type), a polyethyleneoxide-polypropylene oxide (a block type), and a polylactone structureare exemplified.

As the synthesizing methods of the graft copolymer, a macromonomermethod, an ionic polymerization method, a chain transfer method, and amethod of making an active site with radiation and polymerizing amonomer have been conventionally known, but a macromonomer method ispreferably used for the synthesis of the graft copolymer in theinvention for the easiness of the introduction of an ionic functionalgroup into side chain terminals, the facility of the adjustment of thepolymerization degree of a main chain and a side chain, and the easinessof the linking of a main chain and a side chain.

A side chain having an ionic functional group only at one side terminalnot containing to a main chain can also be manufactured by themacromonomer method with a macromonomer having a functional group at theω-position (hereinafter referred to as an ω-position functionalgroup-containing macromonomer) as a starting material.

As the specific examples of the ω-position functional group-containingmacromonomers, ω-hydroxyl macromonomers, such as polyalkylene glycolmono(meth)acrylates, e.g., polyethylene glycol mono(meth)acrylates,polypropylene glycol mono(meth)acrylates, poly(ethylene glycol/propyleneglycol)mono(meth)acrylates, and polyethylene glycol-polypropylene glycolmono(meth)acrylates; and polyester (meth)acrylates, e.g., polylactone(meth)acrylate are exemplified.

Besides the above, ω-carboxyl macromonomers obtained by a method ofreacting (meth)acrylic acid with lactone and acid epoxide, and a methodof reacting α,ω-dicarboxylic acid with an unsaturated monomer having ahydroxyl group are exemplified.

ω-Carboxyl macromonomers can be obtained by the addition of an ionicfunctional monomer by the reaction of ω-hydroxyl macromonomers withcyclic acid anhydride, such as succinic acid anhydride, phthalic acidanhydride, 4-cyclohexene-1,2-dicarboxylic acid anhydride,cis-1,2-cyclohexanedicarboxylic acid anhydride, glutaric acid anhydride,or trimellitic acid anhydride.

It is also possible to prepare a macromonomer having an anionicfunctional group at the ω-position by the addition of an ionicfunctional group, e.g., a sulfonic acid group and a phosphoric acidgroup, via the hydroxyl group of an ω-hydroxyl macromonomer.

Further, the ionic group of the above ω-position ionic group-containingmacromonomer may be cationic. For example, macromonomers having acationic functional group at the ω-position, e.g., ω-amino macromonomersand ω-ammonium macromonomers, can be obtained by the reaction of anω-hydroxyl macromonomer and an ω-carboxyl macromonomer with epoxyaminesor epoxy ammoniums.

As the epoxyamines, glycidyldimethylamine and glycidyldiethylamine areexemplified, and as the epoxy ammoniums, glycidyltrimethylammoniumchloride and triethylglycidylammonium chloride are exemplified.

<2> Main Chain

The main chain constituting the graft copolymer is a polymer comprisinga hydrophobic monomer unit as an essential component and, if necessary,other copolymerizable monomer units as constituents.

(A) Hydrophobic Monomer

As hydrophobic monomers for constituting the main chain of the graftcopolymer, styrene-based monomers, aromatic group-containing monomerssuch as phenyl group-containing (meth)acrylates and phenylgroup-containing maleimides, alkyl (meth)acrylates and the like areexemplified.

The specific examples of the styrene-based monomers include styrene,α-methylstyrene, p-methylstyrene, vinyltoluene and the like.

The specific examples of the phenyl group-containing (meth)acrylatesinclude benzyl(meth)acrylate, phenyl (meth)acrylate,2-hydroxy-3-phenoxypropyl(meth)acrylate, phenoxyethyl(meth)acrylate,phenoxypolyethylene glycol (meth)acrylate, nonylphenyl ethylene oxideadduct (meth)acrylate and the like.

The specific examples of the phenyl group-containing maleimides includeN-phenylmaleimide, N-(2-chlorophenyl)-maleimide and the like.

The specific examples of the alkyl(meth)acrylates includemethyl(meth)acrylate, ethyl(meth)acrylate, propyl (meth)acrylate,butyl(meth)acrylate, hexyl(meth)acrylate, cyclohexyl(meth)acrylate andthe like.

These hydrophobic monomers may be used alone, or two or more monomersmay be used in combination.

(B) Other Monomers

The main chain constituting the graft copolymer contains (A) hydrophobicmonomer unit as an essential component, and can further contain, ifnecessary, other monomer units as constituents. As other monomer units,nonionic monomers other than (A) hydrophobic monomer unit, and ionicmonomers having reverse ionicity to that of the dispersoids such as thelater-described colorants and the like constituting aqueous inks areexemplified.

The specific examples of the nonionic monomers other than (A)hydrophilic monomers include hydroxyalkyl(meth)acrylates such ashydroxyethyl(meth)acrylate and hydroxypropyl(meth)-acrylate;(meth)acrylamide, vinyl acetate, N-vinyl-pyrrolidone,(meth)acrylonitrile, and the like. Crosslinking monomers such asmethylenebisacrylamide, methylenebis-methacrylamide, butanedioldi(meth)acrylate, ethylene glycol di(meth)acrylate, polyethylene glycoldi(meth)acrylate, polypropylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, di(meth)acryloxyethyl phosphate,triallyl cyanurate, triallyl isocyanurate, divinylbenzene, diallylmaleate, polyallyl saccharose, etc., each having two or more vinylgroups in a molecule may further be contained.

As the ionic monomers having reverse ionicity to that of thedispersoids, anionic monomers and cationic monomers are exemplified, andas the anionic monomers, an unsaturated carboxylic acid monomer, anunsaturated sulfonic acid monomer, an unsaturated phosphoric acidmonomer, etc., are exemplified.

The specific examples of the unsaturated carboxylic acid monomersinclude an acrylic acid, a methacrylic acid, a maleic acid, a fumaricacid, a crotonic acid, an itaconic acid, a citraconic acid, etc., andthe anhydrides and salts thereof. The specific examples of theunsaturated sulfonic acid monomers include styrenesulfonic acid,2-acrylamido-2-methylpropanesulfonic acid, 3-sulfopropyl(meth)acrylate,bis(3-sulfopropyl)itaconate, sulfuric acid of 2-hydroxyethyl(meth)acrylic acid, and the salts thereof, etc.

The specific examples of the unsaturated phosphoric acid monomersinclude vinylphosphonic acid, bis(methacryloxy-ethyl) phosphate,diphenyl-2-acryloyloxyethyl phosphate, diphenyl-2-methacryloyloxyethylphosphate, dibutyl-2-acryloyloxyethyl phosphate,dibutyl-2-methacryloyloxyethyl phosphate, etc.

These anionic monomers may be used by one kind alone, or two or moremonomers may be used in combination.

As the cationic monomers, unsaturated tertiary amine-containing monomersand unsaturated ammonium salt-containing monomers are exemplified.

The specific examples of the unsaturated tertiary amine-containingmonomers include monovinylpyridines, e.g., vinyl-pyridine,2-methyl-5-vinylpyridine, and 2-ethyl-5-vinyl-pyridine; styrene having adialkylamino group, e.g., N,N-dimethylaminostyrene andN,N-dimethylaminomethylstyrene; (meth)acrylates having a dialkylaminogroup, e.g., N,N-dimethylaminomethyl acrylate, N,N-dimethylaminoethylacrylate, N,N-diethylaminomethyl acrylate, N,N-diethylamino-ethylacrylate, N,N-dimethylaminopropyl acrylate, and N,N-diethylaminopropylacrylate; vinyl ethers having a dialkyl-amino group, e.g.,2-dimethylaminoethyl vinyl ether; (meth)acrylamides having adialkylamino group, e.g., N—(N′,N′-dimethylaminoethyl)acrylamide,N—(N′,N′-dimethyl-aminoethyl)methacrylamide,N—(N′,N′-diethylaminoethyl)-acrylamide,N—(N′,N′-diethylaminoethyl)methacrylamide,N—N′,N′-dimethylaminopropyl)acrylamide,N—(N′,N′-dimethyl-aminopropyl)methacrylamide,N—(N′,N′-diethylaminopropyl)-acrylamide,N—(N′,N′-diethylaminopropyl)methacrylamide, etc.

As the unsaturated ammonium salt-containing monomers, those obtained bythe quaternarization of the unsaturated tertiary amine-containingmonomers with quaternarization agents such as alkyl halides (alkylgroup: C1 to C18, halogen atom: a chlorine atom, a bromine atom, or aniodine atom); benzyl halides such as benzyl chloride or benzyl bromide;esters of alkylsulfonic acids (alkyl group: C1 to C18) such asmethanesulfonic acid; alkyl esters (alkyl group: C1 to C18) ofarylsulfonic acids such as benzenesulfonic acid and toluenesulfonicacid; dialkyl sulfates (alkyl group: C1 to C4), and the like areexemplified. These cationic monomers may be used alone, or two or moremonomers may be used in combination.

The above other monomers may be used alone, or two or more monomers maybe used in combination.

It is preferred that the ratio of the content of the constituting sidechain in the graft copolymer is from 30 to 99 mass %, and the ratio ofthe content of the main chain is from 70 to 1 mass %, more preferablythe content ratio of the constituting side chain is from 50 to 95 mass%, and the content ratio of the main chain is from 50 to 5 mass %. Whenthe ratio of the content of the main chain is less than 1 mass %, theadsorption onto a dispersoid such as a colorant is insufficient and thegraft copolymer does not function as a dispersant, while when thecontent ratio of the main chain exceeds 70 mass %, crosslinking occursamong the particles of the dispersoid and sometimes the dispersoid isconversely agglomerated. Further, when the content ratio of the sidechain constituting the graft copolymer is less than 30 mass %, asufficient amount of side chains cannot be introduced and electricrepulsion cannot be ensured, so that the graft copolymer does notfunction as a dispersant.

For the synthesis of the graft copolymer, a method of using a radicalpolymerization initiator is preferred for the easiness of polymerizationoperation and molecular weight control, and a solution polymerizationmethod of performing polymerization in an organic solvent is morepreferred, since hydrophobic monomers are hardly soluble in water.

Preferred solvents in performing radical polymerization by solutionpolymerization are ketone solvents, e.g., acetone, methyl ethyl ketone,and methyl isobutyl ketone; acetate solvents, e.g., ethyl acetate andbutyl acetate; aromatic hydrocarbon solvents, e.g., benzene, toluene,and xylene; isopropanol, ethanol, cyclohexane, tetrahydrofuran,dimethylformamide, dimethyl sulfoxide, hexamethylphosphoramide, etc.More preferred solvents are ketone solvents, acetate solvents, andalcohol solvents.

As the radical polymerization initiators, any ordinarily used radicalpolymerization initiators can be used, and specifically ammoniumpersulfate, sodium persulfate, potassium persulfate, peroxy ketal,hydroperoxide, dialkyl peroxide, diacyl peroxide, peroxy dicarbonate,peroxy ester, cyano-based azobisisobutyronitrile,azobis(2-methylbutyronitrile), azobis (2-amidinopropane) hydrochloride,non-cyano-based dimethyl-2,2′-azobisisobutyrate, etc., are exemplified.Organic peroxides and azo compounds capable of easily controllingmolecular weight and having low decomposition temperatures arepreferred, and especially, azo compounds are more preferred.

The use amount of the polymerization initiators is preferably from 1 to15 mass % based on the total mass of the polymerizable monomers, andmore preferably from 5 to 10 mass %.

For controlling the molecular weight of the graft copolymer, anappropriate amount of a chain transfer agent, e.g., mercaptoacetic acid,mercaptopropionic acid, 2-propanethiol, 2-mercaptoethanol, thiophenol,dodecyl mercaptan, or thioglycerol may be added to the polymerization.

The preferred polymerization temperature is from 50 to 150° C., morepreferably from 60 to 100° C. The preferred time of polymerization isfrom 5 to 25 hours. The molecular weight of the graft copolymer of theinvention is preferably from 1,000 to 100,000 as weight averagemolecular weight in the gel permeation chromatography with polystyreneas a standard substance, and is more preferably from 1,000 to 50,000 inview of capable of obtaining proper dispersibility and proper solutionviscosity. A polymer having a weight average molecular weight of lessthan 1,000 results in an insufficient dispersion effect, while when theweight average molecular weight exceeds 100,000, the viscosity increasesto result in non-dissolution and bulk substances occur, so that thecopolymer does not function as a dispersant.

The necessary amount of ionic groups in the graft copolymer ispreferably from 50 to 300 mg-KOH/g in terms of an acid value in the caseof anionic groups, and more preferably from 80 to 250 mg-KOH/g. When theacid value of the copolymer is less than 50 mg-KOH/g, the copolymercannot obtain sufficient electrostatic repulsion, so that the dispersionstability with the lapse of time cannot be obtained, and when the acidvalue exceeds 300 mg-KOH/g, the hydrophilicity of the copolymer itselfbecomes great and the effect as a surfactant lessens, so that thecopolymer does not function as a dispersant.

Further, it is necessary that these anionic groups are partially orentirely ionized by the neutralization with a base. As the bases for usein the neutralization, inorganic alkali agents of alkali metallichydroxides, e.g., potassium hydroxide, sodium hydroxide, and the like,and organic amines, e.g., diethanolamine, triethanolamine, and the likeare exemplified.

[2] Aqueous Ink

The aqueous ink of the invention contains a colorant and an aqueousmedium as essential components in addition to the dispersant describedin the above item [1].

The colorants for use in the aqueous ink of the invention are either ofdyes and pigments insoluble in aqueous solvents.

Dyes are classified into water-soluble dyes and hydrophobic dyes, andhydrophobic dyes are preferred from the viewpoint of water resistance.Thus, as the colorants, hydrophobic dyes or pigments insoluble inaqueous solvents are preferred, and pigments are more preferred in viewof weather resistance.

As hydrophobic dyes, oil dyes, disperse dyes, and the like areexemplified. They can be preferably used both in a water dispersion ofpolymer particles obtained by the incorporation into polymer particlesand in an aqueous dispersion dispersed in water with a dispersant. Thespecific examples of the oil dyes include C.I. solvent Black, C.I.solvent Yellow, C.I. solvent Red, C.I. solvent Violet, C.I. solventBlue, C.I. solvent Green, C.I. solvent Orange, etc. As the dispersedyes, C.I. dispersion Yellow, C.I. dispersion Orange, C.I. dispersionRed, C.I. dispersion Violet, C.I. dispersion Green, etc., can beexemplified.

The pigments may be any of inorganic pigments and organic pigments. Theycan be used alone or in combination, according to necessity.

The range of the dispersed particle size of the pigments is preferablyfrom 0.005 to 15 μm, more preferably from 0.05 to 5 μm, and mostpreferably from 0.01 to 1 μm. When the dispersed particle size is 0.005μm or less, a more amount of a dispersant is required for stabilizingdispersion, so that the performance of the pigments themselves cannot beexhibited, and when the dispersed particle size is 15 μm or more, it isdifficult to maintain a stable dispersion state.

As the inorganic pigments, carbon black, metallic oxides, e.g., titaniumoxide, metallic sulfides, metallic chlorides, etc., can be exemplified.Of these inorganic pigments, especially in black aqueous inks, carbonblack is preferred. As carbon blacks, furnace black, thermal lamp black,acetylene black, channel black, etc., can be exemplified. Specifically,No. 2300, No. 900, MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, No.2200B (all manufactured by Mitsubishi Chemical Corporation), RAVEN1255(manufactured by Columbia), REGAL400R, REGAL330R, REGAL660R, MOGUL L(all manufactured by Cabot), Color Black FW1, Color Black FW18, ColorBlack S160, Color Black S170, Color Black S150, Color Black FW200, ColorBlack FW2, Color Black FW2V, Printex 35, Printex 75, Printex L6, Printex95, Printex U (all manufactured by Degussa), and the like areexemplified.

As the organic pigments, azo pigments, diazo pigments, phthalocyaninepigments, quinacridone pigments, isoindolinone pigments, dioxazinepigments, perylene pigments, perynone pigments, thioindigo pigments,anthraquinone pigments, quinophthalone pigments, etc., are exemplified.

The specific examples of the organic pigments include yellow pigments,e.g., C.I. Pigment Yellow 1, C.I. Pigment Yellow 2, C.I. Pigment Yellow3, C.I. Pigment Yellow 13, C.I. Pigment Yellow 16, C.I. Pigment Yellow74, C.I. Pigment Yellow 83, C.I. Pigment Yellow 128, etc., magentapigments, e.g., C.I. Pigment Red 5, C.I. Pigment Red 7, C.I. Pigment Red12, C.I. Pigment Red 48 (Ca), C.I. Pigment Red 48 (Mn), C.I. Pigment Red57 (Ca), C.I. Pigment Red 112, C.I. Pigment Red 122, etc., and cyanpigments, e.g., C.I. Pigment Blue 1, C.I. Pigment Blue 2, C.I. PigmentBlue 3, C.I. Pigment Blue 15:3, C.I. Pigment Blue 16, C.I. Pigment Blue22, C.I. Vat Blue 4, C.I. Vat Blue 6, etc.

As the aqueous medium, a mixed solvent comprising water and awater-soluble organic solvent is preferred. Water is preferably ionexchange water (deionized water). As the water-soluble organic solvents,alkyl alcohols having from 1 to 4 carbon atoms, e.g., methyl alcohol,ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol,sec-butyl alcohol, tert-butyl alcohol, etc.; amides, e.g.,dimethylformamide, dimethylacetamides, etc.; ketones or keto-alcohols,e.g., acetone and diacetone alcohol, etc.; ethers, e.g.,tetrahydrofuran, dioxane, etc.; polyalkylene glycols, e.g., polyethyleneglycol, polypropylene glycol, etc.; alkylene glycols wherein thealkylene group contains from 2 to 6 carbon atoms, e.g., ethylene glycol,propylene glycol, butylene glycol, triethylene glycol, 1,2-hexanediol,1,2,6-hexanetriol, thiodiglycol, hexylene glycol, diethylene glycol,etc.; glycerol; lower alkyl ethers of polyhydric alcohols, e.g.,ethylene glycol monomethyl (or ethyl) ether, diethylene glycol methyl(or ethyl) ether, triethylene glycol monomethyl (or ethyl) ether, etc.;N-methyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone,etc., are exemplified. Of these water-soluble organic solvents,polyhydric alcohols, e.g., diethylene glycol, etc. and lower alkylethers of polyhydric alcohols, e.g., triethylene glycol monomethyl (orbutyl) ether are preferred.

Incidentally, these water-soluble organic solvents can be used alone orin combination of two or more, if necessary.

Conventionally known various additives that are usually added to aqueousinks may be added to the aqueous ink of the invention as othercomponents besides the above dispersant, colorant, and aqueous medium.As such additives, a surfactant, a defoaming agent, an antiseptic, aprecipitation inhibitor, a chelating agent, a thickener, ananticorrosive, an antioxidant, and the like are exemplified. As thesurfactants, anionic surfactants, e.g., fatty acid salts, higher alcoholsulfates, liquid fatty oil sulfates, alkylaryl sulfonates, etc.,nonionic surfactants, e.g., polyoxyethylene alkyl ethers,polyoxyethylene alkyl esters, polyoxyethylene sorbitan alkyl esters,acetylene alcohols, acetylene glycols, etc., cationic surfactants, e.g.,alkylamines, quaternary ammonium salts, etc., and ampholyticsurfactants, e.g., alkylbetaine, alkylamine oxide, etc., areexemplified.

The amount of the dispersants contained in the aqueous ink of theinvention is preferably from 0.1 to 30 mass % based on the total mass ofthe ink, and more preferably from 1 to 15 mass %. The amount of thecolorants contained in the aqueous ink is preferably from 0.1 to 30 mass%, and more preferably from 1 to 15 mass %.

The ratio of the dispersant to the colorant contained in the aqueous inkof the invention is preferably from 1:1 to 1:30 (ratio by mass), andmore preferably from 1:2 to 1:15.

The amount of the water-soluble organic solvent contained in the aqueousink is preferably in the range of from 3 to 50 mass % based on the totalmass of the aqueous ink, and more preferably in the range of from 3 to40 mass %. Further, the amount of water contained in the aqueous ink ofthe invention is preferably in the range of from 10 to 90 mass %, andmore preferably in the range of from 30 to 80 mass %.

Further, the pH of the aqueous ink is preferably in the range of from 5to 10. By controlling the pH to this range, the solubility of thedispersant comprising the above graft copolymer can be improved and thepreservation stability thereof can be enhanced. Additionally, thecorrosion of the members of an apparatus (e.g., an inkjet recordingapparatus) to which the aqueous ink is applied can be restrained.

For controlling the pH of the aqueous ink, inorganic alkali agents suchas alkali metal hydroxides, e.g., sodium hydroxide and potassiumhydroxide, organic amines, e.g., diethanolamine and triethanolamine,organic acids, e.g., citric acid and tartaric acid, mineral acids, e.g.,hydrochloric acid and phosphoric acid can be used as pH controllingagents.

The aqueous ink according to the invention is obtained by the mixtureand dispersion of each of the above components. The above components canbe used in aqueous inks for inkjet recording and aqueous inks forwriting tools such as aqueous ball point pens and marker pens bydiluting the components with an aqueous medium with or without adding adrying inhibitor and other additives, according to necessity. In thiscase, in order to prevent an inkjet nozzle or a pen tip from clogging bydrying, it is preferred to add a low volatile or nonvolatile solvent ofthe above water-soluble organic solvents. Further, in order to enhancepermeation into a recording medium, it is preferred that a volatilesolvent is added. In particular, in the case where the ink is used inaqueous inks for inkjet recording, it is also preferred that asurfactant is added in order to impart appropriate surface tension tothe ink.

As the dispersing machine for use in the dispersion, a ball mill, a rollmill, a sandmill, and the like are exemplified. Of these, a high speedsand mill is preferred, and the examples of dispersing machines includeSuper Mill, Sand Grinder, Bead Mill, Agitator Mill, Grain Mill,Dyno-Mill, Pearl Mill, Co Ball Mill (all of these are trade names), etc.

The advantages according to the invention are described below.

According to the dispersant of the invention, a colorant, such as apigment that is a dispersoid can be stably dispersed in an aqueousmedium. This is presumed due to the fact that the roles of adsorbingperformance onto a pigment and the affinity with a dispersion medium canbe assigned to the side chain and the main chain, and each performancecan be exhibited at its maximum without interfering with each other.

EXAMPLE

The invention will be described more specifically with reference toExamples and Comparative Examples. In the examples “%” and “parts”respectively mean “mass %” and “mass parts” unless otherwise indicated.

Example 1

<Synthesis of Macromonomer>

A glass flask equipped with a stirrer, an air suction tube and athermometer was charged with 100 g (0.36 mol equivalent) of polyethyleneglycol monomethacrylate (trade name: Blenmer PE200, manufactured byNippon Oils and Fats Co., Ltd.), and 36 g (0.36 mol) of succinic acidanhydride. With bubbling the air and maintaining the inner temperatureof the reaction vessel at 100° C., the mixture was subjected to reactionfor 5 hours, whereby macromonomer 1 having a carboxyl group at theω-position, a polyethylene oxide skeleton, and α,β-ethylenic unsaturateddouble bond was obtained. When the molecular weight of the copolymer wasmeasured by a GPC method with tetrahydrofuran as an eluent, the weightaverage molecular weight (hereinafter abbreviated to Mw) was found to be550 in terms of polystyrene.

Incidentally, Blenmer PE200 is a registered trademark of Nippon Oils andFats Co., Ltd.

<Synthesis of Copolymer>

Subsequently, a glass flask equipped with a stirrer, a dropping funnel,a reflux condenser, a nitrogen gas suction tube, and a thermometer wascharged with 80 g of the above macromonomer 1, 20 g of styrene, and 400g of methyl ethyl ketone. Under a nitrogen current, while maintainingthe inner temperature of the reaction vessel at 78° C., 9 g ofazobisisobutyronitrile (AIBN) was added thereto, followed bypolymerization for 7 hours, whereby a methyl ethyl ketone solution of acopolymer (solids content: 20%) was obtained. Subsequently, potassiumhydroxide equimolar to the carboxyl group contained in the copolymer andion exchange water were added to the above methyl ethyl ketone solutionof the copolymer to neutralize the solution. The methyl ethyl ketone wasdistilled off under reduced pressure to effect solvent removal, wherebyan aqueous solution of copolymer 1 (Mw: 7,600) (solids content: 20%) wasobtained.

<Preparation of Aqueous Ink>

After 15 parts of the above aqueous solution of copolymer 1 (solidscontent: 20%), 15 parts of carbon black (FW-18, manufactured byDegussa), and 70 parts of ion exchange water were mixed and premixingwas carried out for 10 minutes, dispersion treatment was performed onthe following conditions.

Dispersing machine: Sand Grinder (manufactured by Igarashi Kikai)

Pulverization medium: zirconium beads (diameter of beads: 0.3 mm)

Packing rate of the pulverization medium: 50% (by volume)

Pulverization time: 3 hours

After the above dispersion treatment, centrifugation treatment (12,000rpm, 20 minutes) was carried out to remove coarse particles and 40 partsof the resulting dispersion was mixed with 5 parts of glycerol, 15 partsof diethylene glycol, 2 parts of 2-pyrrolidone, and 38 parts of ionexchange water. The pH of the mixture was then controlled to 8 to 10with diethanolamine, and the resulting mixture was filtrated through amembrane filter having a pore diameter of 5.0 μm to thereby obtainobjective aqueous ink 1.

Example 2

Macromonomer 2 (Mw: 600) having a carboxyl group at the ω-position, apolycaprolactone skeleton, and an α,β-ethylenic unsaturated double bondwas obtained in the same manner as in Example 1, except for changing thestarting materials to 100 g (0.40 mol equivalent) ofpolycaprolactone-modified monomethacrylate (trade name: Praxel FM1,manufactured by Daicel Chemical Industries), and 40 g (0.40 mol) ofsuccinic acid anhydride.

Incidentally, Praxel FM1 is a registered trademark of Daicel ChemicalIndustries.

After that, an aqueous solution of copolymer 2 (Mw: 8,000) was obtainedin the same manner as in Example 1, except for changing macromonomer 1used in the synthesis of the copolymer in Example 1 to macromonomer 2.Further, aqueous ink 2 was obtained according to the same procedure asin Example 1.

Example 3

A glass flask equipped with a stirrer, a dropping funnel, a refluxcondenser, and a thermometer was charged with 14 parts of acrylic acid,8 parts of p-toluenesulfonic acid monohydrate, and 0.08 parts ofhydroquinone monomethyl ether as a polymerization inhibitor. Whilemaintaining the inner temperature of the reaction vessel at 80° C., 114parts of ε-caprolactone was dripped over 4 hours. After completion ofdripping, the reaction was continued for further 2 hours at the sametemperature, whereby macromonomer 3 (Mw: 400) having a carboxyl group atthe ω-position, a polycaprolactone skeleton, and an α,β-unsaturateddouble bond was obtained.

After that, an aqueous solution of copolymer 3 (Mw: 7,500) was obtainedin the same manner as in Example 1, except for changing macromonomer 1used in the synthesis of the copolymer in Example 1 to macromonomer 3.Further, aqueous ink 3 was obtained according to the same procedure asin Example 1.

Example 4

In Example 1, macromonomer 1 used in the synthesis of the copolymer waschanged to 80 g (about 0.29 mol equivalent) of polyethylene glycolmonomethacrylate (trade name: Blenmer PE200, manufactured by Nippon Oilsand Fats Co., Ltd.), and a glass flask equipped with a stirrer, adropping funnel, a reflux condenser, a nitrogen gas suction tube, and athermometer was charged with 20 g of styrene, and 400 g of methyl ethylketone. Under a nitrogen current, while maintaining the innertemperature of the reaction vessel at 78° C., 9 g ofazobisisobutyronitrile (AIBN) was added thereto, followed bypolymerization for 7 hours, whereby a methyl ethyl ketone solution of acopolymer was obtained. After that, 28 g (0.28 mol) of succinic acidanhydride was added thereto while maintaining the temperature in thereaction vessel at 78° C., followed by reaction for 15 hours, whereby amethyl ethyl ketone solution of copolymer 4 (Mw: 9,500) was obtained.Aqueous ink 4 was obtained in the same manner as in Example 1, exceptfor changing the neutralization base in the carboxyl group frompotassium hydroxide to triethanolamine.

Comparative Example 1

In Example 1, macromonomer 1 used in the synthesis of the copolymer waschanged to 80 g of polyethylene glycol monomethacrylate (trade name:Blenmer PE200, manufactured by Nippon Oils and Fats Co., Ltd.), and aglass flask equipped with a stirrer, a dropping funnel, a refluxcondenser, a nitrogen gas suction tube, and a thermometer was chargedwith 20 g of styrene, and 400 g of methyl ethyl ketone. Under a nitrogencurrent, while maintaining the inner temperature of the reaction vesselat 78° C., 9 g of azobisisobutyronitrile (AIBN) was added thereto,followed by polymerization for 7 hours, whereby a methyl ethyl ketonesolution of a copolymer was obtained. Subsequently, ion exchange waterwas added to the above methyl ethyl ketone solution of the copolymer.The methyl ethyl ketone was distilled off under reduced pressure toeffect solvent removal, whereby an aqueous solution of copolymer 5 (Mw:9,000) was obtained. Further, aqueous ink 5 was obtained according tothe same procedure as in Example 1.

Comparative Example 2

A methyl ethyl ketone solution of copolymer 6 (Mw: 9,000) was obtainedaccording to the same procedure as in Example 1, except for using 80 gof polycaprolactone-modified monomethacrylate (trade name: Praxel FM1,manufactured by Daicel Chemical Industries) in place of macromonomer 1used in the synthesis of the copolymer in Example 1. However, thecopolymer could not be soluble.

Comparative Example 3

An aqueous solution of copolymer 7 (Mw: 8,500) was obtained according tothe same procedure as in Example 1, except for using 80 g of methacrylicacid in place of macromonomer 1 used in the synthesis of the copolymerin Example 1. Further, aqueous ink 7 was obtained according to the sameprocedure as in Example 1.

Comparative Example 4

An aqueous solution of copolymer 8 (Mw: 8,500) was obtained according tothe same procedure as in Example 1, except for using 80 g of succinicacid 2-methacryloyloxyethyl methacrylate in place of macromonomer 1 usedin the synthesis of the copolymer in Example 1. Aqueous ink 8 wasobtained in the same manner as in Example 1, except for changing theneutralization base in the carboxyl group from potassium hydroxide totriethanolamine.

Evaluation:

Each of the inks obtained above was evaluated for the following items(a) to (d). The results obtained are shown in Table 1 below.

(a) Particle Size of Aqueous Ink:

After each of the obtained aqueous ink was diluted 100 times by theaddition of water, the particle size of the aqueous ink was measuredwith “Microtrack UPA250”, (manufactured by Nikkiso Co., Ltd.).

(B) Dispersion Stability of Aqueous Dispersion:

After each of the obtained aqueous ink was left on standing in a closedstate at 60° C. for 6 months, the aqueous ink was evaluated as failure“x” or good “∘” whether the agglomeration of pigment particles andthickening occurred or did not occur, respectively.

(c) Print Density:

Solid-print was performed on PPC recycled paper (manufactured by NihonKako Seishi) with Micro-Bubble-Jet Printer (Model BJ-10VL, manufacturedby Canon Inc.). Then, optical density of the print after naturalseasoning at room temperature for 24 hours was measured with Macbethdensitometer RD918 (manufactured by Macbeth).

Micro-Bubble-Jet Printer is a registered trademark of Canon Inc.

(d) Picture Quality Characteristics:

With regard to the occurrence of uneven image density and white lines atblack solid image printing, no occurrence was graded “∘”, slightobservation graded “Δ”, and obvious occurrence as “x”. TABLE 1 VolumeAverage Particle Size Dispersion Print Picture Quality (nm) StabilityDensity Characteristics Example 1 100 ◯ 1.40 ◯ Example 2 95 ◯ 1.41 ◯Example 3 97 ◯ 1.39 ◯ Example 4 110 ◯ 1.39 ◯ Comparative 98 X 1.18 ◯Example 1 Comparative — — — — Example 2 Comparative 190 X 1.09 X Example3 Comparative 150 X 1.11 X Example 4

As shown in Table 1, satisfactory results were obtained with the aqueousinks in the Examples as compared with those in the Comparative Examples.It is presumably due to the fact that the dispersion stability of thedispersant (graft copolymer) used in each Example is excellent ascompared with those in the Comparative Examples. The dispersant itselfcould not be soluble in Comparative Example 2, so that evaluation wasimpossible.

INDUSTRIAL APPLICABILITY

The aqueous inks in the invention exhibit no agglomeration of thepigment that is a dispersoid and increase in viscosity of the inks evenwhen left on standing for a long period of time, and show high picturedensity of printed articles, and are excellent in picture qualitycharacteristics, so that the aqueous inks in the invention are extremelyuseful for inkjet recording.

1. Aqueous ink comprising, as a dispersant, a graft copolymer having aweight average molecular weight of from 1,000 to 100,000 and comprising:a polymer having a weight average molecular weight of from 300 to 10,000represented by the following formula (1) as a side chain; and a polymeras a main chain comprising a hydrophobic monomer unit as an essentialcomponent, and, if necessary, other copolymerizable monomer unit as aconstitutional component:

wherein the sum of m and n is 1 or more, p is 0 or 1 or more, and R is achemical structural unit having an ionic functional group.
 2. Theaqueous ink as claimed in claim 1, wherein the ionic functional grouprepresented by R in formula (1) is a carboxyl group, a sulfonic acidgroup, a phosphoric acid group, a primary amino group, a secondary aminogroup, a tertiary amino group, or a quaternary ammonium salt group. 3.Aqueous ink comprising, as a dispersant, a graft copolymer having aweight average molecular weight of from 1,000 to 100,000 and comprising:a polymer having a weight average molecular weight of from 300 to 10,000represented by the following formula (1) as a side chain; and a polymeras a main chain comprising a hydrophobic monomer unit as an essentialcomponent, and, if necessary, other copolymerizable monomer unit as aconstitutional component,

wherein q is 1 or more.
 4. The aqueous ink as claimed in any of claims 1to 3, wherein the side chain in the graft copolymer is formed of amacromonomer.